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We perform analysis of the three-dimensional kinematics of Milky Way diskstars in mono-age populations. We focus on stars between Galactocentricdistances of $R=6$ and 14 \,kpc, selected from the combined LAMOST DR4 redclump giant stars and Gaia DR2 proper motion catalogue. We confirm the 3Dasymmetrical motions of recent works and provide time tagging of the Galacticouter disk asymmetrical motions near the anticenter direction out toGalactocentric distances of 14\,kpc. Radial Galactocentric motions reach valuesup to 10 km s$^{-1}$, depending on the age of the population, and present anorth-south asymmetry in the region corresponding to density and velocitysubstructures that were sensitive to the perturbations in the early 6 \,Gyr.After that time, the disk stars in this asymmetrical structure have becomekinematically hotter, and are thus not sensitive to perturbations, and we findthe structure is a relatively younger population. With quantitative analysis,we find stars both above and below the plane at $R\gtrsim 9$ kpc that exhibitbending mode motions of which the sensitive duration is around 8 \,Gyr. Wespeculate that the in-plane asymmetries might not be mainly caused by a fastrotating bar, intrinsically elliptical outer disk, secular expansion of thedisk, or streams. Spiral arm dynamics, out-of-equilibrium models, minor mergersor others are important contributors. Vertical motions might be dominated bybending and breathing modes induced by complicated inner or externalperturbers. It is likely that many of these mechanisms are coupled together.

monthly notices of the royal astronomical society

Mapping the Galactic disk with the LAMOST and Gaia Red clump sample. II.  3D asymmetrical kinematics of mono-age populations in the disk between 6$-14$  \,kpc